Increasing demand for mobile and personal communication services has renewed interest in spectrally efficient modulation schemes. Since the most efficient forms of RF power amplification are non-linear, CPM (Continuous Phase Modulation) schemes have been preferred for portable wireless applications (e.g Gaussian Minimum Shift Keying--GMSK). Because of growing pressures for extra capacity however, the advantages of retaining a constant envelope are giving way to linear modulation (e.g .pi./4 Shift DQPSK).
Although the filtering applied in linear modulation schemes generally produces gains in spectrum utilisation, envelope variations are also introduced. Such signals when passed through non-linear RF amplifiers undergo distortion (Amplitude-to-Amplitude Modulation & Amplitude-to-Phase Modulation [AM/AM & AM/PM]) which results in a spreading of the spectrum beyond the allocated channel and reduced performance. The inefficiency of conventional linear RF amplifiers (e.g Class A operated under appropriate back-off) would have a disastrous effect on the battery life of portable wireless transmitting equipment. Improvements in RF amplifier efficiency would directly lead to improvements in the talk-time, recharging intervals, and size and weight of the overall wireless unit. The ideal amplifier for linear modulated portable systems is therefore a linear amplifier which is also power efficient.
Because linear amplifiers can amplify signals with any combination of amplitude and phase modulation, the choice of modulation scheme is not limited by the transmitter and hence can be software selectable. This is advantageous in military applications, and in commercial applications crossing international borders and standards. Other applications for linear amplification include various digital cellular and Private Mobile Radio (PMR) systems, traditional Amplitude Modulation (AM) and Single Side-Band (SSB) systems, and instances where transmitter turn-on is to be well controlled such as bandlimited pulse systems found in radar and paging applications and in Time Division Multiple Access (TDMA) systems. Linear Amplifiers can also accommodate the envelope variations caused by the combination of multiple signals such as those found in multicarrier basestations.
Feedforward linearization is one way non-linear but efficient amplifiers can be linearized. It is based on cancelling the distortion of the amplifier at the output. The distortion or error signal is measured by comparing the amplifier output signal with the input. After suitable scaling and delay matching this error signal, which is out of phase with the distortion, is applied to the output resulting in a reduction in the distortion. The error signal must be amplified by a linear RF power amplifier however. This results in a compromise since in general, as the efficiency of an RF power amplifier increases, so does its distortion and hence the error signal level to be amplified by the linear amplifier. The larger the error signal level the larger the linear amplifier and hence the greater the power consumption and the lower the efficiency. Such systems have been applied particularly for wideband linearization schemes. A typical example is disclosed by P. B. Kenington, M. A. Beach, A. Bateman and J. P. McGeehan in PCT Pat. No. WO 91/16760.
It has long been known that feedback can linearize non-linear systems. Cartesian Feedback for example, which uses negative feedback of the baseband quadrature modulation provides excellent reduction in intermodulation distortion with low complexity and cost. A typical example of what is achievable with cartesian feedback is given in by M. Johannson and T. Mattsson, "Transmitter Linearization using Cartesian Feedback for Linear TDMA Modulation", in the proceedings of the 41st IEEE Vehicular Technology Conference, St. Louis, USA, VTC-91, pp 439-444, May, 1991. Other typical feedback techniques include Polar Feedback discussed in U.S. Pat. No. 5,023,937; IF (Intermediate Frequency) Feedback demonstrated by K. G. Voyce and J. H. McCandless, "Power Amplifier Linearization Using IF Feedback", in the IEEE MTT-S digest, pp 863-866, 1989; and RF Feedback demonstrated by H. A. Rosen and A. T. Owens, "Power Amplifier Linearity Studies for SSB Transmissions", IEEE Transactions on Communications Systems, pp 150-159, June, 1964.
High level modulation of the power supply of an RF power amplifier, for the purposes of amplitude (or envelope) modulation, is a well established technique which provides good power efficiency when combined with efficient high level power supply modulators. This is the basis for amplitude feedback only techniques of EP Pat. No. 431201 FIG. 19 (also WO 9100653); of U.S. Pat. No. 3,900,823 FIGS. 1 and 2; of K. Chiba, To Nojima and S. Tomisato, "Linearized Saturation Amplifier with Bidirectional Control for Digital Mobile Radio", in the proceedings of Globecomm (IEEE), pp 1958-1962, 1990; and of M. J. Koch and R. E. Fisher, "A High Efficiency 835MHz Linear Power Amplifier for Digital Cellular Telephony", in the proceedings of the 39th IEEE Vehicular Technology Conference, USA, VTC89, pp 17-18, May, 1989. The problem with these systems is the relatively larger bandwidths of the amplitude signals compared with corresponding baseband quadrature modulation and the inability to control AM-PM distortion. Consequently the spectral control is generally inferior to that of Cartesian Feedback, IF Feedback and RF Feedback. Full Polar Feedback (both Amplitude and Phase feedback) overcomes the limitation of the inability to control AM-PM distortion, but still suffers from the bandwidth expansion problem on both the amplitude and phase signals.
Dynamic control of the DC gate bias has also been shown to improve amplifier efficiency by A. A. M. Saleh and D. C. Cox, "Improving the Power-Added Efficiency of FET Amplifiers Operating with Varying Envelope Signals", IEEE Transactions on Microwave Theory and Techniques, Vol. 31, January 1983. U.S. Pat. No. 4631491 also demonstrates that feedback can be used to control the collector and base bias in a Bipolar Junction Transistor based RF amplifier to improve the efficiency of the amplifier.